Outlook 2000 problems with Exchange 2003 server

we are in the process of migrating our nt 4 domain over to server 2003 and active directory.
The issue we are having is with already migrated users to our new domain, who use Outlook 2000 connecting to the Exch 2003 server...New email notifications take forever, or only pop up, when you click in the folder tree of outlook..any suggestions?